Control of mechanical properties and aging characteristics of flat rolled steel product



8 1969 M. H. KESSLER ETAL 3,460,997

CONTROL OF MECHANICAL PROPERTIES AND AGING CHARACTERISTICS OF FLAT ROLLED STEEL PRODUCT Filed Nov. 10, 1966 i r MONROE H. KESSLER 1 EDWlN J. GEBHARDT ATTORNEYS United States Patent 3,460,997 CONTROL OF MECHANICAL PROPERTIES AND AGING CHARACTERISTICS OF FLAT ROLLED STEEL PRODUCT Monroe H. Kessler, Steubenville, Ohio, and Edwin J. Gehhardt, Weirton, W. Va., assignors to National Steel Corporation, a corporation of Delaware Filed Nov. 10, 1966, Ser. No. 593,507 Int. Cl. C21d J/74, 1/78 US. Cl. 148--2 12 Claims ABSTRACT OF THE DISCLOSURE SPECIFICATION This invention is concerned with control of mechanical properties and aging characteristics of fiat rolled mild steel.

The phenomenon of strain aging of flat rolled mild steel product is undesirable because of an embrittlement effect brought about spontaneously with the passage of time following plastic deformation. Since some type of plastic deformation, due to skin rolling, coiling, flexing, etc., is practically unavoidable in the production of flat rolled steel product, strain aging and its undesirable affects on the mechanical properties of flat rolled mild steel are inherent unless special steps are taken.

The effect of strain aging is an increase in hardness and yield strength and a reduction in ductility. This has been a problem of great concern to fiat rolled steel producers for many years since performance of their product, especially deep drawing capabilities, depends on these properties. One of the special steps taken to avoid this problem has been aluminum additions, about .3% and higher by weight, to the molten steel. Such aluminum additions reduce the susceptibility of mild steel to strain aging and the resulting product is referred to as aluminum-killed or Stabilized steel.

There are detrirnents however in that aluminum additions increase the price of the product, not only because of the cost of the aluminum, but because of the poor overall yield from cast product to shipped product. Also, during annealing operations, aluminum-killed steels exhibit a slower response rate to grain development resulting in an overall harder product even though strain aging characteristics are improved. In practice, when aluminum-killed steels are not box annealed, continuous-anneal processing must be slowed down in order to produce acceptable product. Further, aluminum-killed steels exhibit alumina streaks which cause surface and other defects when producing flat rolled product.

3,460,997 Patented Aug. 12, 1969 "Ice In spite of these disadvantages, aluminum additions have been the most widely accepted solution to strain aging problems for years in the production of flat rolled steel product. Other types of steel, such as rimmed steel are used where possible to produce more economic flat rolled steel product but at a sacrifice of the desirable strain aging characteristics of aluminum-killed steel.

The present invention makes possible production of fiat rolled steel product including control of mechanical properties and aging characteristics without relying on additional agents, such as aluminum, for stabilizing the steel and without requiring selection of steels based on dilfering casting methods, differing chemistry, etc. Further these control features are available using continuous strip heat treatment processing rather than the more cumbersome and more expensive batch heat treatment processing generally practiced in the prior art for producing drawing quality stock.

The invention teaches processing of very low carbon steel having a carbon content around .01 to .02% C and higher, under some circumstances, but not higher than .03% C. More responsive control of mechanical properties and aging characteristics result from processing this very low carbon steel than was available in the prior art.

Open hearth, basic oxygen, electric, or other suitable steel-making processes may be used for manufacture of the molten mild steel used in the present invention. After preparation, the molten steel is then pneumatically degassed, for example, by vacuum degassing. An important feature of the degassing taught by the present invention is the lowering of carbon content at this early stage in processing. A maximum carbon content of about .02% after degassing is preferred in practice, but carbon content up to about .03% can produce acceptable product with improved mechanical properties and aging characteristics over those available with the processes of the prior art.

The degassed low carbon molten steel is then cast, either continuously into slabs or cast into ingots and formed into slabs. Casting takes place without a rimming action yet a low-inclusion, clean steel is produced, a result not obtainable in the prior art. Also high percentage yields result. For example, when casting in ingot molds, the yield is increased at least 6% over rimmed steel ingots which must be end-cropped to remove impurities and highly segregated metalloids formed at the top of the casting during the rimming action. The increase in yield over aluminum-killed steel is about 10%, or higher, since the aluminum-killed steel slabs must be end-cropped until there is no trace of the piping ordinarily found in the top of aluminum-killed ingots. The elimination of a rimming action brought about by the present invention has a number of additional advantages especially in a continuous casting operation.

After preparation, slabs are converted to flat rolled steel product in a conventional manner by hot rolling to strip gages normally between about .065" and about .185" depending on the desired end use.

In producing cold rolled products, the steel strip is cold rolled after hot rolling to gages required for cold rolled sheets, galvanized sheets, or other coated products, e.g., gages between about .005" and about .125". Such cold rolling can be carried out with conventional equipment by methods well known in the art.

Improved mechanical properties and improved aging characteristics result from heat treatment of the above described low-carbon flat rolled steel product in accordance with the present invention. The treatment includes heating the flat rolled steel product to a temperature at least above its lower critical temperature A and then controllably cooling the product to a temperature about 1200 F. Further, enhanced results are obtained in controllably cooling the product to about 350 F. and surface coloration requirements can extend controlled cooling as low as 200 F. The controlled cooling referred to includes control of cooling atmosphere as Well as temperature.

Heating of the flat rolled product is carried out in a continuous strip operation with the heating taking place as rapidly as possible to provide for uniform grain growth. Preferably the rate of heating would take the steel from approximately room temperature to its lower critical temperature in about one minute, or to its upper critical temperature A in less than a minute and a half, i.e., roughly at a rate between about 10 F. to about 20 F. per second.

Mechanical properties are readily controlled with the low carbon steel of the present invention by the controlled heating above the lower critical temperature and the drawing capability is increased. For example, to produce substantially nonaging, deep drawing quality flat rolled steel product, the strip is heated to a temperature above its upper critical temperature (about 1650 F. for .Ol% carbon steel and about 1630 F. for 02% carbon steel). While to produce steel having aging characteristics and mechanical properties which are improved over those available with commercial flat rolled mild steel product of the prior art, the strip is merely heated above its lower critical temperature (about 1500 F. for .Ol% carbon steel and about 1335 F. for 02% carbon steel.)

In cooling the low carbon steel of the present invention from its upper critical level to its lower critical level the change in temperature is substantially reduced over that required With prior art mild steels. This feature facilitates continuous strip operations taught by the present invention and cooling can be carried out more readily and more rapidly without detriment to grain development or control of mechanical properties. A cooling rate of about 5 to 6 per second is preferred at this stage but cooling rates up to about 10 F. per second produce improved product over that available in the prior art, and rates as low as 2 F. per second are practical for the continuous processing operations of the present invention. Cooling the strip from its lower critical temperature to about 1200 F. is carried out within the same range of about 2 F. to about 10 F. per second.

Cooling the low carbon steel of the present invention from about 1200 F. to a temperature between 700 F. to 900 F. can be carried out as rapidly as possible consistent with maintaining proper atmosphere. Because of the low percentage of carbon in the steel, any carbide entrapped by this rapid cooling and the affect of such carbon entrapment, can be readily cured, without residual quench-aging effects in the steel, by subsequent steps in the process. This is a valuable advantage in-processing coated steel which, in the prior art, often sustained quench-aging affects due to the rapid cooling required by the coating process line. The coating itself, because of its low melting temperature, often prevented desired relief of such quench-aging effects in the prior art.

At a temperature between about 700 F. and 900 F., uncoated flat rolled steel product, produced in accordance with the present invention, is gradually cooled to a temperature of about 350 F., or lower. This is accomplished by coiling the product at a temperature between about 700 F. and about 900 F., or higher, and gradually cooling at a rate not exceeding 200 F. per hour.

For production of coated flat rolled steel product, the

strip is controllably cooled to about 1200 F., as described above. Cooling below that temperature can be determined by the coating operation without permanent detriment to the product. After coating the product is heated to a temperature approaching, but below, the melting temperature of the coating and then gradually cooled to about 350 F., or lower, at a rate not exceeding 200 F. per hour. For example, in producing galvanized steel, cold rolled steel strip is heated to a temperature above its lower critical temperature, controllably cooled to about 1200 F., then cooled at a rate determined by the coating line to a temperature required by the coating process, usually about 300 F. to 1000 F., then coated, then cooled rapidly after coating as required by the coating line, ordinarily to a temperature between F. and F. and then reheated to a temperature in the range of about 400 F. to about 600 F., preferably about 550 F. The galvanized strip, usually in coil form, is then gradually cooled at a rate not exceeding 200 F. per hour. With the teachings of the present invention, the quench-aging efiects which are common with conventional galvanized mlld steel are readily eliminated.

Aluminum coated steel can be reheated after coating to a temperature about 650 F. and higher, and then gradually cooled at a rate not exceeding 200 F. per hour. Coating process lines and coating metals which permit coiling after coating at a temperature in the range of 700 to 900 without damage to the coating do not require a reheating step after coating and can follow the controlled cooling steps taught above for cold rolled sheet.

A distinct advantage of the present invention 1s that the heat treatment after the coating processing can be substantially less in time and temperature, than that required with prior art steels to reduce quench-aging effects caused by the coating process.

While controlled cooling to about 350 F. is referred to above, controlled cooling can extend below that level in order to control surface coloration of uncoated product. For example, mechanical properties of .Ol% carbon steel in cold rolled sheet form are determined by controlled cooling to about 350 F. However, removing this uncoated product from the controlled atmosphere, which is mildly reducing to iron oxide, will result in a blue surface color. If the same product is removed from the controlled atmosphere at about 250 F., a straw surface color will result. Therefore, cooling under controlled conditions of temperature and atmosphere can be extended to 200 F., and lower, dependent on surface coloration requirements.

The present invention provides better, more practical production control of mechanical properties of flat rolled product than was available in the prior art and improves aging characteristics of that product. For example, conventional galvanized mild steel has a Rockwell B hardness number between about 50 and '60 and ages such that an increase in hardness of 1020% takes place during normal storage. Low carbon steel processed in accordance with the present invention, with control of cooling to about 1200 F., with the galvanizing operation controlling subsequent cooling and without subsequent heat treatment, has a Rockwell B hardness number between about 40 and 50 and ages such that an increase in hardness of about 5 to 10% takes place during normal storage time.

A conventional low temperature box anneal of the above conventional product, i.e., heat treatment at about 550 F. for approximately four hours followed by furnace cooling will lower the Rockwell B hardness number of the conventional galvanized mild steel by about five to ten points, but that product will age the same 10 to 20%. On the other hand, by carrying out the post coating heat treatment of the present invention, the low carbon product described above has its Rockwell B hardness number lowered by 5 to 10% and the aging characteristics of this product are such that the hardness will increase by about only 5% with passage of time in excess of normal storage tlme.

A cold rolled sheet product, treated in accordance with the present invention by heating above its upper critical level and controlling cooling will have drawing quality equal to, or better than, the highest grade drawing quality aluminum-killed steel which has been box annealed and otherwise treated in accordance with the prior art. In addition, the novel product of the present invention will have uniformity of texture, non-scalloping drawing qualities and improved surface because of the absence of alumina streaks and the absence of a grit-rolling surface required for box annealing. This product of the present invention will also be substantially nonaging. These advantages are in addition to the low cost available due to the continuous processing of the present invention as opposed to the batch handling processes of the prior art.

The accompanying drawings show schematically novel continuous processing apparatus for treating uncoated flat rolled steel product in accordance with the present invention.

Steel from coils 12 and 14 is formed into continuous strip 16 at welder 18. The continuous strip then passes through cleaning section 20. The clean, continuous strip is thereafter maintained in a controlled atmosphere during treatment, the controlled atmosphere being at least mildly reducing to iron oxide.

In heating furnace 22 the strip is heated rapidly at about to per second to a temperature above its lower critical level, or higher, to a temperature above its upper critical level.

In cooling section 24, which follows heating furnace 22, the strip is cooled to about 1200 F. at a rate between about 2 and about 10 F. per second.

The strip is then cooled from about 1200 F. to a temperature in the range of 700 to 900 F. in cooling section 26, which follows cooling section 24. The strip is cooled in section 26 as rapidly as control of the atmosphere will permit.

The strip travels from cooling section 26 into coiling chamber 30 which also has a controlled atmosphere. The atmosphere in chamber 30 is at least mildly reducing to iron oxide. If heat is required to maintain coiling section 30 at desired temperature a complete-combustion heating system can be used. The continuous strip is coiled at station 32 or 34 from which it is lowered to a level with cooling tunnel 36. The strip is coiled at a temperature between 700 and 900 F. and introduced at about coiling temperature into the entry chamber 38 of cooling tunnel 36. The double door arrangement for chamber 38 permits separate control of the atmospheres of coiling chamber 30 and cooling tunnel 36.

The strip is moved through the cooling tunnel on conveyor 37 and cooled gradually at a rate not exceeding 200 F. per hour. With the strip in coil form, this may require forced cooling which is provided by tubes 40 located at the upper side of cooling tunnel 36. At the exit side of the cooling tunnel an exit chamber 42 is provided with a double door arrangement for maintaining desired atmosphere within the cooling tunnel.

The cooling required in cooling section 24 to bring the strip to a temperature around 1200 F. will largely determine the line speed. Other operations will be keyed to this speed. Looping towers can be used before and after the cleaning section as required. Also travel through heating section 22 and cooling sections 24 and 26 can be by multiple passes.

Cooling tunnel 36 is located beneath the remainder of the line for purposes of saving space and other advantages. Also separate atmosphere control means, including inlet and outlet valves for the furnace means and various cooling means are shown. It will be understood that other arrangements such as cooling tubes can be used within the teachings of the present invention.

In addition to elimination of the cumbersome and costly batch handling processes utilized in the prior art, a steel mill, using the present invention, can standardize its steel manufacturing processes and its steel chemistry. That is, the various specifications for steel sheets and strip, included coated steel sheets and strip can be produced with teachings of the present invention since precise control of mechanical properties and aging characteristics is readily available. The economic advantages of eliminating various types of steel, aluminum-killed, rimmed, etc., and of eliminating batch heat process and apparatus will be obvious to those skilled in the art.

While a significant contribution of the present invention involves processes and products which effectively supplant aluminum-killed steels by more economic production of flat rolled steel with better control of mechanical properties and aging characteristics, the use of exothermic materials, which may include aluminum in amounts around .005 by weight, to maintain bath temperature during vacuum degassing, does not preclude such a process or its product from the scope of the present invention. Similarly, degassing as a part of a casting operation would fall within the scope of the present invention. Other variations of specifically described steps are possible while relying on the basic teachings of the present invention, therefore the scope of the present invention is to be determined from the appended claims.

What is claimed is:

1. Metallurgical process for improving mechanical properties and aging characteristics of flat rolled steel comprising (A) providing low carbon molten mild steel,

(B) pneumatically degassing the low carbon molten steel with carbon content of the molten steel after degassing being no higher than about .03

(C) solidifying the molten steel,

(1D) rolling the solidified steel to produce flat rolled steel product,

(E) heating the flat rolled steel product to a temperature at least in excess of the steels lower critical temperature, and then (F) controllably cooling the flat rolled steel product to about 1200 F. at a cooling rate between about 2 F. and about 10 F. per second.

2. The process of claim 1 in which the carbon content of the molten steel after degassing does not exceed .02%.

3. The process of claim 1 in which the cooling rate of step (F) is about 5 F. per second.

4. The process of claim 1 in which step (D) includes hot rolling followed by cold rolling to desired cold rolled strip gage.

5. The process of claim 1 in which step (E) includes heating to a temperature above the upper critical temperature for the steel.

6. The process of claim 1 in which steps (E) and (F) are carried out in a continuous-strip operation with the heating rate of step (E) being between about 10 F. and about 20 F. per second.

7. The process of claim 6 in which steps (E) and (F) include controlling the atmosphere contacted by the fiat rolled steel product to be at least mildly reducing to iron oxide.

8. The process of claim 7 in which the flat rolled steel product is cooled from about 1200 F. to between 700 F. and 900 F. at the fastest cooling rate practical while maintaining proper atmosphere control.

9. The process of claim 8 in which the flat rolled steel product is coiled at a temperature between 700 F. and 900 F. and cooled gradually to a temperature about 350 F. at an average rate not exceeding about 200 F. per hour, while maintaining the atmosphere contacted by the coiled flat rolled product to be at least mildly reducing to iron oxide.

10. The process of claim 4 in which the cold rolled strip is coated with a protective metal after step (E) in a continuous strip coating operation in which cooling of the cold rolled strip from about 1200" F. to a temperature below the coating temperature for the cold rolled strip is determined by the continuous strip coating operation.

11. The process of claim 10 in which the coated cold rolled strip is heated after metal coating to a temperature approaching, but less than, the melting temperature of the protective metal coating in a range between about 400 F. and about 800 F., followed by gradual cooling of the coated cold rolled strip to about 350 F. at an average rate not exceeding 200 F. per hour.

12. The process of claim 4 in which the cold rolled strip is heated to a temperature of about 1650 F., cooled to temperature about 1200" F. at an average rate in the range of 2 F. to 10 F. per second, cooled at any rate from about 1200 F. to about 800 F., and then cooled References Cited UNITED STATES PATENTS 10 JOHN F. CAMPBELL, Primary Examiner PAUL M. COHEN, Assistant Examiner US. Cl. X.R.

gradually from about 800 F. to about 200 F. at an aver- 15 144 age rate not exceeding 200 F. per hour. 

